This article introduces what is intracavity frequency doubling and its advantages and applications。

Frequency doubling, like many other nonlinear frequency conversion effects, only has high frequency conversion efficiency when the light intensity hitting the nonlinear crystal material is large enough. This is difficult to achieve with low average power, especially with continuous light.
Intracavity frequency doubling is an effective method to solve this problem. In this method, the frequency doubling crystal is placed in the laser cavity (or the cavity of the optical parametric oscillator).

The improvement of the frequency doubling efficiency of this method mainly comes from the following two aspects:

1. In the resonator, the optical power and intensity will be much stronger, which can greatly improve the frequency doubling efficiency in a single pass, usually by an order of magnitude;
   Although the frequency doubling efficiency of a single pass is still not very high, since the unconverted pump light is still in the cavity, the final frequency doubling efficiency will be greatly improved.
2. Therefore, it is possible to generate a high-power frequency-doubled light by using intra-cavity frequency doubling, and the power of the frequency-doubling light is not much lower than that of the non-frequency-doubling light directly output without a frequency-doubling crystal.

The intracavity frequency-doubling resonator must contain a dichroic mirror, which has high transmittance for the frequency-doubling beam, so as to output the frequency-doubling light. All resonator mirrors in the cavity have high reflectivity for fundamental frequency light. (The original output beam splitter will no longer be used). For a linear resonator, frequency doubling will be counterproductive in both directions of propagation. The two beams can be combined into one beam. If the end mirrors have no parasitic losses and have ideal relative phase changes, the conversion efficiency of intracavity frequency doubling of a linear cavity can be four times that of a single pass.

Regarding polarization and phase matching, there are also different options:

• Frequency doubling of type I phase matching can be used in polarized emission lasers.
• Type II phase-matched frequency multipliers are suitable for lasers emitted by unpolarized light.

In both cases, the frequency-doubled light is linearly polarized.

Many continuous-light green and blue laser sources are based on intracavity frequency doubling, and such lasers are already available that generate output powers of tens of watts. This technique is also applicable to red lasers, which utilize vanadium lasers at 1342 nm.

There are some negative issues with using intracavity frequency doubling for CW lasers. One of the very annoying problems is that in some cases lasers can have significant intensity noise. This is caused by the nonlinear effect of the resonator mode, which is not only affected by nonlinear frequency transformation, but also by spatial hole burning and higher-order resonant modes. Polarization issues also need attention, especially for the case of type II phase matching. For different cases, its instability can be eliminated by some different laser design techniques. For example, use a relatively long resonant cavity (increasing the number of resonant modes), or stable single-frequency modes. The thermal effect of nonlinear crystals is usually not a big problem, because the thermal effect is much smaller than that of laser crystals.

If the gain bandwidth is larger than the phase matching bandwidth of the nonlinear crystal, the laser wavelength will be larger than the wavelength region of nonlinear transformation, which will result in very low conversion efficiency. This problem can be eliminated with an intracavity optical filter that exactly matches the wavelength of the nonlinear transformation.

When the crystal temperature is not suitable and the phase matching is not satisfied, so that the frequency doubler does not work, the intracavity power of the fundamental frequency light will become quite high (especially in Q-switched lasers). The design of the laser should ensure that no laser damage will occur.

Only in rare cases are Q-switched or mode-locked lasers used for intracavity frequency doubling. This is because the peak power of such pulsed lasers typically enables high-efficiency conversion, and nonlinear elements can adversely affect pulse formation. In Q-switched lasers, intracavity frequency doubling significantly slows pulse accumulation, and in passively mode-locked lasers, it is difficult to generate ultrashort pulses because it partially cancels the effect of the saturable absorber.

An alternative technique, applicable to both single-frequency and mode-locked lasers, is to utilize a resonant booster cavity outside the laser (see Resonance Frequency Doubling). Usually, only one of the two techniques, intracavity frequency doubling and resonant frequency doubling, is used. However, both techniques are used in Ref. [15]. In this document, a frequency-doubling resonator is placed inside the laser resonator of a fiber laser. In general, these two technologies are not suitable for high-power fiber lasers: the cavity loss of intracavity frequency doubling is too high to achieve efficient frequency doubling, and the system construction and operation of resonance frequency doubling are more complicated. But combining the two approaches gives good results: the long fiber laser resonates strongly with the resonant frequency of the short frequency-doubling resonator, and the resonance enhancement occurs only in nonlinear crystals, not in fibers occurs, therefore, the high loss in the fiber section does not pose a problem.

In addition, other types of nonlinear frequency conversion can also be realized using laser resonators: such as stimulated Raman scattering, sum/difference frequency generation, and optical parametric oscillators.